1. Field of the Invention
The present invention relates to a collision energy absorbing structure of a vehicle which is used for absorbing a collision energy generated by a collision of vehicles such as railroad vehicles by bellows-like deformation (plastic deformation) of a tubular energy absorbing member with rectangular cross section to relieve impact when collision occurs.
2. Description of the Related Art
In general, it is known that a pipe member can be used as an energy absorbing member for absorbing a collision energy by bellows-like deformation, because the pipe member is plastically deformed like bellows while suppressing deformation of the Euler buckling when a compression force is axially applied to the pipe member by suitably selecting a dimension or a thickness thereof. Since such bellows-like deformation is capable of stably absorbing the collision energy, conventionally, the collision energy absorbing structure using the pipe member has been widely used.
By the way, since a high reaction force is generated in triggering the bellows-like deformation when such pipe member is used to form the collision energy absorbing structure, the impact acting on passengers is increased in an initial stage of collision. Accordingly, the following structure has been proposed as a structure for reducing the maximum force in the initial stage of collision.    (1) By way of example, in a structure disclosed in Japanese Patent No. 2650527 in which ribs are longitudinally formed integrally with the inside of a member body extruded to have closed cross section and provided in the longitudinal direction of the vehicle to define a plurality of parts in the member body, the ribs have inclined portions at end portions thereof which are extending from points connecting the ribs to peripheral portions of the member body toward the inside of the body. In this structure, since the inclined portions are thus formed in the ribs at end face portion of the member body and the corresponding rib cross-sectional areas are extremely small, the member body tends to be axially deformed by buckling upon application of an axial collision force to the end portion of the member body in collision of the vehicle. Consequently, an initial reaction force can be reduced.    (2) As another example, as disclosed in Japanese Patent No. 2882243, a plurality of arc-shaped grooves inwardly recessed and axially extending from the front end are formed circumferentially and at substantially equal intervals at the front end portion of a tubular chassis frame provided in the longitudinal direction of the vehicle. In this structure, upon application of impact to the front end portion from front, the arc-shaped groove portions and the portions without the grooves are continuously and axially deformed by buckling while being deformed alternately and inwardly or outwardly from the front end, so that plastic deformation is stabilized with bellows being close to one other.    (3) As a further example, as disclosed in Publication of Examined Patent Application No. Hei. 11-5564, a side member of a vehicle formed to have a hollow shaft by extruding aluminum is provided with at least one rib in the longitudinal direction thereof and the thickness of the rib and the thickness of the side member are gradually increased from the end portion toward a vehicle chamber side in the longitudinal direction of the vehicle. Because of such a gradual increase in the thickness, this structure is capable of reducing an initial maximum force while keeping a collision energy absorbing ability large as the whole.    (4) As a still further example, as disclosed in Publications of Examined Patent Applications Nos. Hei. 9-277953, 9-277954, in an energy absorbing member capable of absorbing a collision energy by bellows-like bucking deformation, a cross section of a buckling deformation starting end is a polygon-shaped closed cross section with angles more than 4 and a cross section of the other end is a polygon-shaped closed cross section having sides more than those of the cross section of the starting end, between which the cross section gradually varies. These structures enable the increasing of the buckling force and the reducing of the initial impact force by utilizing the polygon-shaped cross section or a tapered shape with varying cross section to suppress the initial impact force or stabilize the first buckling deformation.
However, the structures of (1)-(4) suffer from the following drawbacks.    (A) In the structure disclosed in Japanese Patent No. 2650527, since the inclined portions are formed at the ribs inside of the member body, its structure is complicated. In addition, this structure is only applicable to the structure having inside ribs (e.g., extruded aluminum).    (B) The structure disclosed in Japanese Patent No. 2882243 is applicable only to the cylindrical frame. Since the bellows-like deformation tends to be unstable in the cylindrical frame as compared to the tubular frame with rectangular cross section, a stable energy absorbing characteristic is difficult to obtain.    (C) In the structures disclosed in Publications of Examined Patent Applications Nos. Hei. 11-5564, 9-277953, 9-277954, since the pipe member (side member or energy absorbing member) has a structure with the cross section varying in the axial direction thereof, a special and complicated process is needed.